Method for recovering rare earth metal

Abstract

A method for recovering a rare earth metal according to one embodiment of the present invention comprises a crushing step, a dissolution step, a first separation step, a second separation step, and a recovery step. In the crushing step, a rare earth magnet is crushed. In the dissolution step, a rare earth metal is dissolved in liquid magnesium metal from the magnet powder obtained in the crushing step. In the first separation step, the residue in the dissolution step and the liquid magnesium metal that contains the rare earth metal are separated from each other. In the second separation step, the liquid magnesium metal adhering to the residue in the dissolution step, which is separated in the first separation step, is replaced with liquid zinc metal. In the recovery step, the rare earth metal is recovered by distilling each of the liquid magnesium metal that contains the rare earth metal and is separated in the first separation step and the liquid magnesium metal that contains the rare earth metal and is separated in the second separation step.

Description

Method for recovering rare earth metals

[0001] The present invention relates to a method for recovering rare earth metals by separating and recovering rare earth metals from rare earth magnets.

[0002] In recent years, rare earth magnets have been used in many home appliances, electric vehicles (EVs), generators, pump motors, large electric motors, and other applications, and demand for rare earth magnets is on the rise. Meanwhile, rare earth elements, such as neodymium (Nd), praseodymium (Pr), terbium (Tb), and dysprosium (Dy), are valuable resources, and because they are heavily dependent on imports, it can be difficult to ensure a stable supply due to changes in social conditions. For this reason, there is a need to establish recycling technologies for magnet shavings generated in magnet manufacturing plants, as well as recycling technologies for separating and recovering rare earth elements from rare earth magnets collected from the market.

[0003] For example, molten metal extraction is known as a technique for separating and recovering rare earth metals from rare earth magnets. For example, Patent Document 1 discloses a technique that utilizes the property that Fe (iron) and B (boron), which make up neodymium magnets, are almost insoluble in Mg (magnesium), to dissolve the rare earth elements in the neodymium magnet in molten metallic magnesium, and then evaporate the magnesium in which the rare earth elements have dissolved, thereby separating and recovering the rare earth elements.

[0004] Furthermore, Patent Document 2 discloses that the weight of magnesium added is 5 to 15 times the weight of the rare earth magnet added, with the aim of recovering rare earth metals from rare earth magnets with high efficiency by molten metal extraction. Furthermore, Patent Document 3 discloses a technology for separating rare earth metals from an alloy containing Dy and Tb as constituent metals without melting and extracting the rare earth metals.

[0005] Patent No. 5709164 Patent No. 5942718 Patent No. 6179699

[0006] In order to establish recycling technology for rare earth magnets, it is necessary to further improve the efficiency of recovering rare earth metals from rare earth magnets. For example, in the method of separating and recovering rare earth metals using the above-mentioned molten metal extraction technology, if it were possible to further recover rare earth metals from the magnetic material that is the residue in the separation process between the magnet from which the rare earth metals have been extracted and the metallic magnesium in which the rare earth metals have been dissolved, it is expected that the efficiency of recovering rare earth metals from rare earth magnets can be greatly improved.

[0007] In view of the above circumstances, an object of the present invention is to provide a method for recovering rare earth metals that can improve the efficiency of recovering rare earth metals from rare earth magnets.

[0008] A method for recovering rare earth metals according to one embodiment of the present invention includes a crushing step, a dissolving step, a first separation step, a second separation step, and a recovery step. The crushing step crushes rare earth magnets. The dissolving step dissolves rare earth metals from the magnet powder obtained in the crushing step into liquid metallic magnesium. The first separation step separates the residue from the dissolving step from liquid metallic magnesium containing rare earth metals. The second separation step replaces the liquid metallic magnesium containing rare earth metals attached to the residue from the dissolving step separated in the first separation step with liquid metallic zinc. The recovery step recovers rare earth metals by distilling the liquid metallic magnesium containing rare earth metals separated in the first separation step and the second separation step, respectively.

[0009] According to one embodiment of the method for recovering rare earth metals of the present invention, rare earth metals dissolved in the liquid metallic magnesium separated in the first separation step and in the liquid metallic magnesium separated in the second separation step are recovered, thereby improving the efficiency of recovering rare earth metals from rare earth magnets.

[0010] The crushing step may include a treatment for hydrogen embrittlement of the rare earth magnet. Because grain boundary fracture occurs in the crushed rare earth magnet, rare earth metals are exposed on the particle surfaces. This increases the efficiency of dissolving rare earth metals into liquid metallic magnesium. In particular, the hydrogen embrittlement treatment causes the volume of the rare earth magnet to expand, resulting in internal collapse, thereby shortening the crushing time.

[0011] The average particle size of the magnet powder is, for example, 1 μm or more and 150 μm or less, more preferably 50 μm or more and 75 μm or less, and the maximum particle size is 90 μm or less.

[0012] The weight ratio of the liquid metallic magnesium to the magnet powder in the dissolving step is, for example, 0.36 or more and 5 or less. The weight ratio of the liquid metallic magnesium to the magnet powder may be 1 or more and 2 or less. According to the present technology, the rare earth magnet is pulverized through a crushing step, so that the amount of liquid metallic magnesium used can be reduced.

[0013] The atmosphere in the dissolving step may be an atmospheric pressure or a reduced pressure atmosphere, and the temperature of the liquid metallic magnesium may be 650°C or higher and 1000°C or lower.

[0014] The dissolution treatment may involve dissolving the rare earth metal from the magnet powder into the liquid metallic magnesium while fluidizing (e.g., stirring or bubbling) a mixture of the magnet powder and the liquid metallic magnesium in a furnace.

[0015] The recovery step may include a process of separating light rare earth elements and heavy rare earth elements from the rare earth metal by drifting them due to differences in melting points, thereby enabling the light rare earth elements and heavy rare earth elements to be recovered separately.

[0016] In this case, the recovery step may involve distilling the liquid metallic magnesium containing the rare earth metal, and then heating the residue to a temperature equal to or higher than the melting point of Nd, thereby bleeding the liquid light rare earth element from the solid heavy rare earth element-containing material.

[0017] The method may further include a heating step of heating the magnet powder obtained in the crushing step at a temperature of 400°C or higher and 650°C or lower before the dissolving step, and a step of recovering rare earth metal oxides floating on the surface of the liquid metallic magnesium after the dissolving step.

[0018] The rare earth metal oxide may be a Dy oxide or a Tb oxide.

[0019] According to the present invention, it is possible to improve the efficiency of recovering rare earth metals from rare earth magnets.

[0020] 1 is a basic process flow illustrating a method for recovering rare earth metals according to one embodiment of the present invention; FIG. 2 is a conceptual diagram illustrating a method for separating and recovering rare earth metals by melt extraction; FIG. 3 is a diagram illustrating an example of the temperature dependence of the diffusion coefficient of Nd relative to Mg; FIG. 4 is an experimental result showing the relationship between the average particle size of a magnet material and the dissolution time of a rare earth metal; FIG. 5 is a diagram comparing the melting points and vapor pressures of metallic magnesium and rare earth metals (Nd, Pr, Dy, Tb); FIG. 6 is a schematic diagram illustrating a method for recovering rare earth metals from metallic magnesium containing rare earth metals, and further separating and recovering light rare earth elements and heavy rare earth elements from the rare earth metals; and FIG. 7 is a process flow illustrating the details of the processing procedure of the method for recovering rare earth metals according to this embodiment.

[0021] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0022] [Outline of Treatment Flow] Fig. 1 is a basic process flow illustrating a method for recovering rare earth metals according to one embodiment of the present invention. The method for recovering rare earth metals according to this embodiment includes a receiving step (ST101), a crushing step (ST102), a dissolving step (ST103), a first separation step (ST104), a second separation step (ST105), and a recovery step (ST106).

[0023] The receiving process (ST101) receives magnet shavings generated in magnet manufacturing plants or rare earth magnets collected from the market, and performs various processes such as sorting rare earth magnets suitable for further processing and demagnetizing the magnetic materials. Details of these processes will be described later.

[0024] In the crushing step (ST102), rare earth magnets collected from the market are crushed. This crushing step is performed for the purpose of facilitating the dissolution of rare earth metals from the magnetic material into molten magnesium metal in the melting and extraction process in the melting step (ST103). More specifically, the melting and extraction process involves crushing the magnetic material to an optimal particle size so that the rare earth metals present in the solid phase can be easily extracted.

[0025] In the dissolving step (ST103), rare earth metals are dissolved in liquid metallic magnesium from the magnet powder obtained in the crushing step (ST102). As will be described later in this embodiment, the rare earth metals are dissolved in the metallic magnesium by melt extraction while the magnet powder and metallic magnesium mixed in a predetermined weight ratio in a container are flowed (for example, stirred or bubbling) in a melting furnace.

[0026] In the first separation step (ST104), which is performed after the melting step (ST103), the magnet powder from which the rare earth metal has been extracted, which is the residue from the melting step, is separated from the liquid metallic magnesium in which the rare earth metal has been dissolved (liquid metallic magnesium containing rare earth metal). As will be described later in this embodiment, the container is tilted in the melting furnace to discharge the liquid metallic magnesium in which the rare earth metal has been dissolved from the container.

[0027] The second separation step (ST105) separates the liquid metallic magnesium containing rare earth metals from the residue from the dissolving step (ST103) separated in the first separation step (ST104) by replacing the liquid metallic magnesium containing rare earth metals adhering to the magnet powder from which the rare earth metals have been extracted, with liquid metallic zinc.

[0028] In the recovery step (ST106), the liquid rare earth metal-containing metallic magnesium separated in the first separation step (ST104) and the second separation step (ST105) is distilled to recover the rare earth metal.

[0029] According to this embodiment, in the recovery process, rare earth metals are recovered from the liquid metallic magnesium containing rare earth metals separated in the first separation process and from the liquid metallic magnesium containing rare earth metals separated in the second separation process, respectively, thereby improving the efficiency of recovering rare earth metals from rare earth magnets.

[0030] Each step will be described in detail below.

[0031] [Receiving Process] Magnet shavings generated in magnet manufacturing plants and rare earth magnets collected from the market are typically permanent magnet materials that make up motors or generators built into electrical equipment, EVs (electric vehicles), etc., and are extracted as recovered magnets by dismantling, demagnetizing, and disassembling the products. Permanent magnet materials are generally neodymium-based magnets made of the metallic elements Nd (neodymium), Fe (iron), and B (boron), and neodymium magnets are classified into neodymium sintered magnets and neodymium bonded magnets.

[0032] On the other hand, EVs and large generators require strong magnetic forces (for example, 1 giga gauss or more). EV main shaft motors, in particular, are used at high speeds, which increases the temperature. Therefore, rare earth magnets exist that, in addition to the usual neodymium magnet composition, contain, for example, 0.5% to 5% of heavy rare earth metal elements such as Dy (dysprosium) or Tb (terbium), allowing them to retain their magnetic properties even at high temperatures.

[0033] When large electric motors and the like are assembled, the magnets are not magnetic. They are assembled into motors in a non-magnetic state, and magnetization is performed to ensure magnetism, allowing the motor to perform as designed. When dismantled, the magnets built into the motor remain magnetic, so for workability and safety reasons, the magnets are demagnetized when dismantling. Demagnetization is generally performed by heating the magnet to a high temperature above the Curie point of the magnet (for example, 400°C to 650°C). However, small motors used in home appliances have weak magnetic force, so dismantling can be carried out without demagnetizing.

[0034] As such, much of the magnetic material collected from the city has been demagnetized for dismantling, or crushed (broken) during dismantling. Furthermore, the magnets used in the city are a mixture of ferrite magnets, alnico magnets, neodymium-bonded magnets, samarium-cobalt magnets, and neodymium magnets. Furthermore, some neodymium magnets contain heavy rare earth elements such as Gd (gadolinium), so before they can be supplied to metal refining, it is desirable to sort the magnets and identify their source.

[0035] It is desirable that magnets that have been treated with anti-corrosion coating, especially those with a 20-50 μm coating of Ni (nickel) or Cu (copper), be separated and managed upon receipt, and then sent to a process where the Ni or Cu coating on the magnet surface is removed. Furthermore, if a mill certificate verifying the properties of the material is required, it is desirable to carry out production management that links the preparation of incoming inspection records with manufacturing process records. Magnet shavings generated from the cutting process in magnet manufacturing plants are not subject to the above-mentioned magnet sorting process or anti-corrosion coating removal process, because they are cut before anti-corrosion processing and have a limited magnet composition.

[0036] [Crushing Process] Examples of crushing methods include electric pulse crushing. Grain boundary fracture occurs in the crushed magnet, exposing rare earth metals on the particle surface. If a technique is used that exposes rare earth metals at the interface, the rare earth metals precipitate on the surface during melt extraction with metallic magnesium, and the dissolution time is also short. Other crushing methods include clamping the magnet between two rollers at high pressure to cause grain boundary crushing, or hydrogen embrittlement to cause the magnet to disintegrate from the inside. Crushing methods using hydrogen embrittlement have the advantage of preventing fires from occurring when crushed magnets are crushed. Jet mill crushing using argon gas or helium gas is also applicable.

[0037] Hydrogen embrittlement occurs when a magnet absorbs hydrogen, causing volume expansion and resulting in the magnet shattering from the inside. The reaction formula is as follows: Nd ２ Fe １４ B+4H ２ = 2NdH ２ +12Fe+Fe ２ B Therefore, NdH２ If the dissolution phenomenon is poor, a dehydrogenation reaction is required. ２ = Nd + H ２ (g) DyH ２ = Dy + H ２ (g)

[0038] When calculating the Gibbs free energy of the dehydrogenation reaction for Nd and Dy using Outotec's HSC software, it was found that both Nd and Dy change from an endothermic reaction to an exothermic reaction at 1000°C to 1200°C, and the reaction becomes rapidly more intense. For this reason, it is desirable to carry out the dehydrogenation reaction during the temperature increase operation under reduced pressure. It was estimated that the dehydrogenation reaction proceeds at 600°C to 650°C under reduced pressure.

[0039] [Melting Process] (Apparatus Configuration) Fig. 2 is a conceptual diagram illustrating a method for separating and recovering rare earth metals by melt extraction. The processing apparatus 10 shown in the figure includes a melting furnace 11. Inside the melting furnace 11, there is placed a container 12 containing the powder of the magnetic material produced in the crushing process (ST102) and metallic magnesium in a predetermined weight ratio.

[0040] The melting furnace 11 is a heating furnace capable of heating the interior of the furnace to a temperature equal to or higher than the melting temperature of the metallic magnesium in the container 12. The melting furnace 11 is connected to a vacuum pump 13, which allows the interior of the furnace to be evacuated and maintained at a predetermined reduced pressure. The melting furnace 11 is also connected to a gas source 14 such as argon gas or nitrogen gas, and is configured to be able to maintain the interior of the furnace at this gas atmosphere. The container 12 is made of a material that does not react with magnesium, such as iron or an iron alloy.

[0041] During the melting and extraction of rare earth metals, the interior of the melting furnace 11 is typically degassed and maintained in a non-oxidizing atmosphere, which is further replaced with argon gas or the like. The pressure within the melting furnace 11 may be atmospheric pressure or a predetermined reduced pressure atmosphere.

[0042] A bubbling nozzle 20 or a stirring blade 15 for fluidizing the contents is disposed inside the vessel 12. The stirring blade 15 stirs the mixture M of powdered magnetic material and molten metallic magnesium contained in the vessel 12, promoting the dissolution of rare earth metals from the magnetic material into metallic magnesium. The stirring blade 15 is made of cast iron, and its surface is preferably coated with a ceramic material such as silicon carbide (SiC) or boron nitride (BN). In this embodiment, an electromagnetic stirring mechanism is employed, capable of rotating the stirring blade 15 at a predetermined rotation speed using electromagnetic energy generated by a drive unit (magnet unit) 16 installed outside the bottom of the melting furnace 11, facing the bottom of the vessel 12. The bubbling nozzle 20 is another example of a fluidization means that replaces or supplements the above-mentioned stirring method and is configured to inject argon gas or the like into the vessel 12 from a gas source 14. The bubbling nozzle 20 can also be used in conjunction with the above-mentioned stirring mechanism.

[0043] As shown in Fig. 2, the container 12 is configured to be tiltable within a predetermined angular range around a horizontally extending tilting axis 17. The tilting axis 17 is installed at one end of a support base 18 fixed to the container 12, but is not limited to this and may be installed directly on the side of the container 12 without the support base 18. By tilting the support base 18 together with the container 12 based on a control command from a control unit that controls the processing device 10, it is possible to pour liquid metallic magnesium, in which the rare earth metals in the content M that has undergone melt extraction processing have been dissolved, into an auxiliary container 19 disposed adjacent to the container 12. The auxiliary container 19 is made of a material that does not react with magnesium, like the container 12, such as iron or an iron alloy.

[0044] When metallic magnesium covers the magnet surface and dissolves the rare earth metal, the coefficient (diffusion coefficient) of Nd diffusing into Mg is temperature dependent as shown in Figure 3, but is generally 2.5 x 10E-8 (cm ２ / s). Therefore, it is desirable to melt the magnesium metal at atmospheric pressure at a temperature of 650°C or higher and 1000°C or lower, which is the melting point of the magnesium metal. However, since the vapor pressure of the metal elements is reduced by reducing the pressure inside the melting furnace 11, the melting step may be carried out at a temperature of 700°C or lower.

[0045] (Regarding the particle size of the magnet material) Next, the particle size of the magnet material suitable for the melt extraction process of the rare earth magnet into metallic magnesium will be considered.

[0046] For example, if we assume that the magnet material has been crushed into particles with an average diameter of 1 mm, the surface area of ​​a 1 mm sphere is S = 4πr ２ Therefore, the radius is 0.05 cm, so S = 0.0314 cm ２ It takes about 348 hours for metallic magnesium to penetrate deep into the material. This is an unrealistic value when considering actual operation. Therefore, when considering the optimal crushed particle size of the magnetic material from the perspective of melting and extracting rare earth metals into metallic magnesium, the average particle size of the powder of the magnetic material is preferably 1 μm or more and 150 μm or less, and more preferably 50 μm or more and 75 μm or less.

[0047] Figure 4 shows the results of an experiment that shows the relationship between the average particle size of the magnetic material and the melting time of the rare earth metal. For example, if the diffusion coefficient is kept constant at 2.5 x 10E-8 with respect to temperature for particle sizes between 1 mm and 10 μm, and the average particle size of the pulverized magnet that will melt in about 1 to 2 hours is calculated, it can be seen that the average particle size of the powder is generally between 50 μm and 75 μm, as shown in Figure 4.

[0048] If the average particle size of the powder is 50 μm to 75 μm, large and small particles will exist in a standard normal distribution. However, from the perspective of melt extraction alone, the larger the particle size, the greater the weight of rare earth metals. Therefore, when operating at a constant extraction time, the amount of dissolved and extracted will be small, and the yield of rare earth metals obtained by melt extraction will be extremely poor. From this, it is estimated that in actual operation, the cutoff (applicable upper limit) is limited to 90 μm, and the dissolution time, including the time for precipitation and separation after dissolution, is limited to 3.5 hours. In this case, the residue on the 166 mesh sieve may be circulated again for crushing in the crushing step (ST102), so that it is not subjected to metallic magnesium melt extraction.

[0049] The melting time is preferably between several tens of minutes and several hundred hours. However, a long melting time at high temperatures results in poor energy efficiency per unit magnet processing weight, so the shorter the melting time, the better. Therefore, a melting time of between several tens of minutes and one day is desirable. For example, by making the average particle size of the powder 150 μm or less, the melting time can be reduced to approximately 8 hours or less (see Figure 4). Furthermore, since the melting time can be shortened by making the particle size of the raw material smaller, a melting time of approximately 2 to 5 hours (for example, an average particle size of 75 μm or more and 120 μm or less) is even more desirable.

[0050] Furthermore, the smaller the average particle size of the crushed raw material, the faster the rate at which the rare earth metals in the magnet raw material and the molten metallic magnesium diffuse and penetrate and dissolve from the surface of the magnet. For this reason, the average particle size is preferably 50 μm to 75 μm, but since the average particle size is the average of larger and smaller particles, it is even more desirable to make the particle size 90 μm or less.

[0051] For this reason, the received raw material, regardless of whether it is demagnetized or not, can be subjected to grain boundary crushing using a crushing device such as a pin mill or jet mill, and the crushed raw material can be weighed and filled into a receiving container, after which the receiving container can be placed in a hydrogen embrittlement device and held for a certain period of time in atmospheric or pressurized hydrogen gas, allowing the magnets to expand and fracture. The hydrogen gas pressure should desirably be between atmospheric pressure and 3 atmospheres, for example. For safety reasons, an inert gas such as helium gas can also be used in combination.

[0052] The receiver may be the container 12 (see FIG. 2) used in the processing device 10. In this case, the weight of metallic magnesium is measured and added to the container 12, and the container 12 is then loaded into the melting furnace 11. The interior of the melting furnace 11 is heated until the metallic magnesium reaches its melting point of 650°C, and degassing is carried out under reduced pressure. Hydrogen gas is generated when hydrogen embrittlement occurs at temperatures between 600°C and 650°C, and the reduction reaction of the rare earth metals that make up the magnet is also completed. By sequentially performing these processes of crushing, hydrogen embrittlement, degassing, and heating, the magnet material can be efficiently adjusted to a particle size suitable for the melting process.

[0053] In order to promote the melting and extraction of rare earth metals by metallic magnesium, metallic magnesium must dissolve and penetrate deep into the magnet particles. By removing the air between the crushed magnets by vacuum degassing, it is possible to prevent the melting and extraction of rare earth metals from being hindered by gas (air or inert gas) present on the magnet surface. ２ If the dehydrogenation reaction proceeds in the molten state of metallic magnesium, hydrogen (H ２ ) Since hydrogen gas is released from the molten magnesium metal, it is desirable to release the hydrogen gas before the temperature of the magnesium metal reaches its melting point (650°C).

[0054] Furthermore, in this melting step (ST103), when the rare earth metal dissolves in the molten magnesium metal through diffusion-limited dissolution from the magnet powder surface, the dissolution phenomenon that forms NdMgB or NdMg alloys may remain in the particle surface region. Therefore, by stirring the magnet powder and the molten magnesium metal with stirring blades 15 in container 12, the efficiency of dissolving the rare earth metal in the molten magnesium metal can be improved.

[0055] (Regarding the weight mixing ratio of magnet and metallic magnesium) Next, let's consider the weight mixing ratio between the pulverized raw magnet and metallic magnesium. For example, let's assume that the tap density (T.D.: closest packing density) of the pulverized magnet is a volume ratio of approximately 50 / 50 (50% volume ratio is voids). A pulverized magnet is a finely pulverized product in which the anticorrosion layer of Ni and Cu on the magnet's surface has been peeled off with hydrochloric acid or the like, demagnetized, pulverized, and the surface of Dy and other elements has been reduced with hydrogen. When metallic magnesium replaces these voids, it becomes the closest volume mixing ratio, but stirring is not possible in this state. Therefore, assuming that the fluidity is adjusted by increasing the liquid (magnesium) side, the calculations assumed that the volume ratio at which stirring is possible is 40 / 60.

[0056] Here, 100 cm ３ Considering the mixed melt of 40 cm ３ is a magnet, so if the specific gravity of the magnet is 7, the weight will be 280g. ３becomes metallic magnesium, and assuming the specific gravity of that metallic magnesium is 1.7, the weight is 102g. Adding this together gives 280g + 102g = 382g, and the specific gravity of the mixture of magnet and metallic magnesium can be calculated to be 3.82. Therefore, a volume ratio of 40 / 60 means a weight mixing ratio of 73 / 24. Therefore, if the magnet is 1, the Mg should be mixed at a weight ratio of 0.36 and then sent to the dissolving step (ST103). In other words, it is assumed that the crushed magnet can be sufficiently immersed in magnesium liquid and stirred, as long as the powder surface is degassed.

[0057] However, Nd ２ Fe １４ In the basic composition of B, 26.7% of 280g is Nd, so considering that 26.7% of 280g of magnet, or 74.78g, is Nd, the Nd / Mg ratio is 74.78 / (74.78+102)=0.423, and since only about 42.3% dissolves in the Nd / Mg alloy phase diagram, it can be seen that a slightly larger amount of metallic magnesium may be necessary. From this, the weight mixing ratio when actually conducting a melting extraction test can be determined depending on the packing density of the crushed material, the possibility of stirring, the mixing ratio in the Nd alloy phase diagram, etc., and the weight ratio of metallic magnesium to magnet is 0.36 to 5, more preferably 1 to 2, but the weight ratio required for metallic magnesium to dissolve the magnet (weight ratio of magnesium to magnet) may be 1 or less.

[0058] [First Separation Step] The content M (see FIG. 2) of the container 12 after the dissolving step (ST103) is composed of magnet powder (Fe+B) from which the rare earth metals have been extracted and liquid metallic magnesium (Mg+Nd) in which the rare earth metals have been dissolved. In the first separation step, after non-contact stirring of the mixed liquid by the stirring blade 15 is stopped, the container 12 (support base 18) is tilted by a predetermined angle around the tilting axis 17 as shown in FIG. 2, and the liquid metallic magnesium in which the rare earth metals have been dissolved is poured from the container 12 into the auxiliary container 19. In this way, the magnet powder from which the rare earth metals have been extracted (residue from the dissolving step) and the liquid metallic magnesium in which the rare earth metals have been dissolved (liquid metallic magnesium containing rare earth metals) are separated.

[0059] In this first separation step (ST104), the contents M may be poured into the auxiliary container 19 through a filter (strainer) that allows only liquid metallic magnesium in which rare earth metals have been dissolved to pass through, so as to prevent solid magnet powder from flowing into the auxiliary container 19.

[0060] Let us assume that approximately 99% of the rare earth metals are dissolved in metallic magnesium by performing melt extraction of the rare earth metals in molten magnesium at 700°C with non-contact stirring for approximately two hours. In this case, Fe and B settle out and separate due to the difference in specific gravity as solid undissolved residues in the molten magnesium.

[0061] The weight of the magnet, 280g, is calculated as follows: Nd ２ Fe １４ Therefore, the mass is 2 moles of Nd = 144.3 x 2 = 288.6 g / mol, 14 moles of Fe = 55.8 x 14 = 781.2 g / mol, 1 mole of B = 10.8 g / mol, so the total mass of 1080.6 g / mol corresponds to a weight of 280 g, and by multiplying this by 0.2591, the weight correction values ​​are Nd = 74.78 g, Fe = 202.42 g, B = 2.8 g, and the Nd content weight ratio can be calculated as 26.7%.

[0062] Since the liquid (Mg + Nd) is 102g + 74.78g = 176.78g and the solid (Fe + B) is 202.42g + 2.8g = 205.22g, the liquid weight ratio can be calculated as 176.78 / 382 = 46wt% and the solid weight ratio as 54wt%. Furthermore, the liquid volume ratio is 103 and the solid volume ratio is 29.3, so 103 / (103 + 29.3) = 77vol%, and the solid volume ratio is 23vol%. Therefore, the magnesium metal molten metal with the dissolved rare earth metals can be separated. Assuming that the settling rates of Fe and B can be calculated using the Stokes equation, even if the particle size is about 10µm, the difference in specific gravity is large, so it is expected that they will settle after several hours of standing.

[0063] Next, the weight of the liquid lost along with the separated Fe+B solid is calculated. In the above example, the volume ratio of the liquid (Mg+Nd) is 77 vol %, weighing 176.78 g, and the volume ratio of the solid (Fe+B) is 23 vol %, weighing 205.22 g. However, 23% of the liquid is lost with the solid, resulting in a total of 40.65 g. The 40.65 g of liquid (Mg+Nd) lost with the solid contains a 42.3% Nd concentration, resulting in a loss of 40.65 × 0.423 = 17.19 g of Nd compared to the total Nd amount of 74.78 g. Therefore, in this embodiment, the solid residue after solid-liquid separation, i.e., the mixture of Fe and B, is not discarded as is, but rather Mg is replaced from the residue to recover Nd (second separation step (ST105)).

[0064] [Second Separation Step] In the second separation step, for example, a separation method utilizing a difference in specific gravity is employed as a method for separating Mg from the residue from the dissolution step (ST103) separated in the first separation step (ST104). This method may involve centrifugation to minimize the loss of the MgNd liquid along with the solid, or a method in which gas is blown onto the liquid side on a ceramic perforated tray to minimize the amount of liquid adhering to the surface of the residue. A non-metallic inclusion filter for molten aluminum may also be used. In this embodiment, as the second separation step (ST105), liquid rare earth metal-containing metallic magnesium (hereinafter referred to as rare earth metal-containing metallic magnesium) adhering to the residue from the dissolution step separated in the first separation step is replaced with liquid metallic zinc, thereby separating the rare earth metal-containing metallic magnesium from the residue.

[0065] After the first separation step, Zn (zinc) is added to the container 12 containing the magnet powder from which the rare earth metals have been extracted and the rare earth-containing magnesium metal attached to its surface. The amount of Zn to be added is not particularly limited, and may be, for example, an amount that can fill the surface of the powder from which the rare earth metals have been extracted with Zn.

[0066] Next, the container 12 is reheated to a temperature at which Zn and Mg melt in the melting furnace 11. As a result, Mg rises to the top of the molten metal because of its low specific gravity, and zinc fills the voids in the Fe-B, replacing Mg. According to the phase diagram, Mg and Zn do not alloy and remain as Mg up to 6.2%, so Mg separates as described above due to the difference in specific gravity.

[0067] The Mg can be recovered in a separate container (such as a ladle or crucible) by tilting the container 12, and the zinc can be recovered from the Fe-B-Zn in a zinc distillation furnace. Since the melting rate of rare earth metals is slower than that of Mg, if the temperature is rapidly raised to the melting point of Zn and the time required for replacement with Mg is shortened, the rare earth metals dissolved in the Mg will not transfer to Zn. The recovered molten Mg may be contained in the auxiliary container 19, or in a container separate from the auxiliary container 19.

[0068] [Recovery Step] In the recovery step (ST106), the rare earth metal-containing magnesium metal separated in the first separation step (ST104) and the rare earth metal-containing magnesium metal separated in the second separation step (ST105) are distilled to recover the rare earth metal.

[0069] In this embodiment, a crucible (which may be, for example, auxiliary container 19 in FIG. 2 ) containing rare-earth metal-containing metallic magnesium is placed in a distillation furnace to evaporate the Mg, and the resulting residue, the rare-earth metal, is recovered. To evaporate the Mg, a multi-stage precision distillation or a highly energy-efficient VCD (Vapor Compressed Distillation) type heating furnace can be used. Furthermore, in this embodiment, rare-earth metals such as Nd and Dy are then evaporated and recovered from the residue. Specifically, light rare-earth elements such as Nd and Pr and heavy rare-earth elements such as Dy and Tb are separated from the rare-earth metals by drifting due to differences in their melting points.

[0070] Figure 5 shows a comparison of the melting points and vapor pressures of metallic magnesium and rare earth metals (Nd, Pr, Dy, and Tb). Theoretically, metallic magnesium can be separated from the rare earth metals by distillation at a temperature between 650°C and 1000°C, preferably between 800°C and 900°C. The remaining rare earth metals can be separated by raising the temperature above the melting points of Pr (931°C) and Nd (1021°C), preferably around 1100°C, so that Nd and Pr bleed out in liquid form from the solid Nd / Pr / Dy / Tb mixture of light and heavy rare earths (bleeding).

[0071] 6 is a schematic diagram illustrating a method for recovering rare earth metals from rare earth metal-containing magnesium metal, and further separating and recovering light rare earth elements and heavy rare earth elements from the rare earth metals. For example, after physical separation of Fe and B, the rare earth metal-containing magnesium metal (Mg + REE) is left to settle, allowing the dissolved rare earth metals to settle. At this time, it may be cooled to below the melting point of metallic magnesium, which is 650°C or less. The solid state is placed in a distillation furnace, and the metallic magnesium is distilled at 650°C to 1000°C. The temperature of the residue is raised to 1100°C, allowing the liquefied light rare earths Nd and Pr to be separated from the solid heavy rare earths Dy and Tb.

[0072] The metallic magnesium separated in the gaseous phase by distillation is recycled and reused as a dissolving medium in the dissolving step (ST103). Meanwhile, the rare earth mixed metal (REE) remaining in the solid phase by distillation is separated by bleeding as a light rare earth mixed metal of Nd and Pr, and the final remaining solid Dy and Tb can be recovered as a rare earth mixed metal.

[0073] At 1100°C, Dy and Tb are below their melting points, so the rare earths remaining after distillation precipitate as solids, but didymium Di (Nd-Pr alloy) is above its melting point and remains in a molten state. Therefore, if the container is held at 1100°C for a specified time, a layer of didymium separates at the top, and solid Dy and Tb precipitate at the bottom. If the container is cooled in this state, the heavy rare earth metals Dy and Tb can be easily separated from the light rare earth metal didymium.

[0074] The separated light rare earth metal (didymium Di) may be reused as an Nd-Pr alloy, or Nd and Pr can be recovered separately by distilling Nd and Pr in that order, taking advantage of the difference in vapor pressure. Similarly, the separated heavy rare earth metals (Dy, Tb) can be recovered separately by distilling Dy and Tb in that order, taking advantage of the difference in vapor pressure.

[0075] [Regarding pretreatment of magnet material after crushing] In order to easily dissolve rare earth elements in metallic magnesium, it is desirable that the rare earth elements are metallic. More specifically, in order to more efficiently melt and extract rare earth metals, it is necessary to devise a method of not converting the magnet raw material into rare earth oxides, or to convert only heavy rare earth elements, etc. into oxides and then dissolve them.

[0076] The Ellingham diagram is commonly used to compare the degree of oxidation of metals. However, the Gibbs free energy of rare earth oxides is not well known. For example, Figure 2 on page 3 of "The Production of Rare Earth Based Magnesium and Aluminum - Alloys A Review" by Ahmad Rizky Rhamdani, Muhammad Akbar Rhamdhani, Geoffrey Brooks, Mark I. Pownceby, Yudi Nugraha Thaha, Trevor B. Abbott, John Grandfield & Chris Hartley, Mineral Processing and Extractive Metallurgy Review, An International Journal, Published online: 08 Aug 2023, shows the relationship between the Gibbs free energy and temperature during the oxidation of rare earth metal elements.

[0077] According to the above literature, when comparing Mg and rare earth elements at the same temperature, the smaller the ΔGγ value of an element, the more easily it oxidizes. When arranging the metal elements used in neodymium magnets, it is shown that the heavy rare earth metals Tb and Dy are oxidized first, followed by the light rare earth metals Sm, Nd, and Pr, and then Mg. Furthermore, the light rare earth oxides from Sm to Nd and Pr to La are easily reduced in molten Mg and dissolved in Mg, and Dy oxide can be reduced in molten Mg, but Tb oxide is very stable, making it difficult to reduce from its oxide during Mg melting.

[0078] This suggests that magnets require pretreatment to minimize oxidation or that processing methods should be developed to ensure elemental forms that dissolve easily in molten magnesium. In other words, magnet manufacturing requires a magnetic production method in which metal elements are homogeneously dispersed in an Fe alloy to create NdFeB magnetic grain boundaries, and heavy rare earth elements such as Dy and Tb are segregated to the grain boundary surfaces to provide heat resistance. Therefore, it is easy to infer that rare earth elements in magnet shavings generated at magnet factories remain as metallic elements until they are dissolved in magnesium, which makes them easily soluble in molten magnesium. However, magnets used in the market or collected through recycling may have oxidized or been crushed by being left in the environment, and the possibility of them converting to rare earth oxides through reaction with oxygen over time cannot be ruled out. Therefore, it is believed that magnets collected through recycling require processing methods and operations different from those used for magnet scraps from magnet manufacturing factories.

[0079] Considering this background, the "metal magnesium melting, extraction and distillation processing method," which enables direct metal smelting from magnet shavings in magnet factories and neodymium magnets collected from the city, simplifies many manufacturing processes and is also desirable from the perspective of ensuring manufacturing quality. It is therefore extremely significant to consider and develop a technology that does not use any fluorine gas, a greenhouse gas.

[0080] Therefore, in this embodiment, the magnets are first crushed in the crushing step (ST102), and then the rare earth metal components are analyzed using non-destructive testing or fluorescent X-ray diffraction. By analyzing in advance whether the Dy element is Dy metal or Dy oxide, or whether the Tb element is Tb metal or Tb oxide, the conditions for the subsequent magnesium dissolution operation can be changed. For example, if it is found that the Dy or Tb supported on the magnet grain boundaries exists as oxides, the oxides are actively floated and separated and recovered using the following method.

[0081] First, before the crushed magnets are melted in a magnesium melting tank, non-destructive testing or fluorescent X-ray diffraction is used to analyze whether the Dy element is Dy metal or Dy oxide, and the operating conditions for the subsequent magnesium melting process are changed. If there is a large amount of Dy oxide, the crushed powder is subjected to a roasting process at 400°C to 650°C, which was performed during the demagnetization process. Because magnets are easily combustible, they are roasted uniformly in a short period of time using an inert gas while monitoring the oxygen partial pressure, and only the heavy rare earth elements that are easily oxidized, such as Dy and Tb, are oxidized.

[0082] Furthermore, at the end of the stirring of the molten metal in the magnesium melting tank, i.e., after the light rare earth elements Nd and Pr have been fully dissolved in the magnesium, heavy rare earth oxides (such as Tb oxide) in the molten magnesium are floated by bubbling with an inert gas, and the heavy rare earth oxides floating on the surface of the molten magnesium are scraped off and recovered using a skimmer (scraper) or the like.

[0083] For example, if we assume that the average particle size of the Fe and B that make up the main body of NdFeB is reduced to about 50 μm through the milling process, the particle size of the rare earth elements in the magnet will be about 2 μm when produced by the SC (strip casting) method. In this case, Nd and Pr are easily reduced with Mg and dissolved in the molten Mg, but Dy oxides and Tb oxides are difficult to dissolve in the molten Mg, so they are raised to the surface of the molten Mg by bubbling or other methods. On the other hand, it is easy to predict that Fe, which has a high specific gravity, will settle to the bottom of the molten metal, and B will exist in the intermediate layer between Mg and Fe.

[0084] In this way, by focusing on the phenomenon that heavy rare earth oxides are difficult to dissolve in molten Mg and can float, it is possible to separate expensive heavy rare earths in the form of oxides on the surface of molten Mg. Meanwhile, the recovered material floating on the surface of the molten Mg (such as Tb oxide) can be separately fluorinated to form rare earth fluorides, which can then be converted into metallic Dy and metallic Tb by Ca thermal reduction, allowing them to be used in new magnets. It is also possible to sell them as heavy rare earth oxides.

[0085] In addition, when the flotation treatment of heavy rare earth oxides as described above is assumed, Dy and Tb, which are easily oxidized after pulverization of the magnetic material, may be actively oxidized with water vapor, oxygen, etc. In this case, the process of non-destructive testing or non-destructive component analysis by fluorescent X-ray diffraction can be omitted. However, since the pulverized magnetic material is highly reactive, safety precautions are required.

[0086] [Details of Processing Procedure] FIG. 7 is a process flow showing details of the processing procedure of the method for recovering rare earth metals in this embodiment.

[0087] First, rare earth magnets are crushed to a predetermined particle size (e.g., average particle diameter of 50 μm to 75 μm) using a pin mill or the like (ST201). The crushed magnets are then subjected to non-destructive component analysis using methods such as fluorescent X-ray diffraction to determine the presence or absence of oxides of heavy rare earth metals (Dy, Tb) in the magnets (ST202). Next, the crushed magnet powder is refined by hydrogen embrittlement (ST203), and the powder and metallic magnesium are then placed in a container 12 (see FIG. 3) in a predetermined weight ratio (e.g., a weight ratio of Mg to magnet of 5 or less), and then placed in the melting furnace 11 (ST204).

[0088] Next, the inside of the melting furnace 11 is replaced with an argon gas atmosphere at normal pressure or reduced pressure, and then the temperature is heated to 650°C to 1000°C, and the magnet powder and metallic magnesium are stirred with the stirring blades 15, thereby reducing the magnet elements and melting and extracting them into metallic magnesium (melting step, ST205). Thereafter, if the presence of oxides of heavy rare earth elements is confirmed in the non-destructive component analysis step (ST202), the Dy oxides and Tb oxides that float to the surface of the molten metal are recovered (ST206).

[0089] 2, the magnet powder from which the rare earth metal has been extracted and the metallic magnesium liquid in which the rare earth metal has been dissolved are separated (first separation step, ST207). As a result, the magnet from which the rare earth metal has been extracted remains as a residue in the container 12, and the metallic magnesium liquid in which the rare earth metal has been dissolved is transferred to the auxiliary container 19.

[0090] Next, the rare earth metal-containing metallic magnesium in auxiliary vessel 19 is distilled in a distillation furnace to recover the rare earth metal (ST208). The distilled metallic magnesium is solidified and then reused as a magnesium raw material in the melting step (ST204).

[0091] On the other hand, for the magnet powder (Fe-B) remaining in the container 12, metallic zinc is added, and then metallic zinc and metallic magnesium are heated and dissolved, and the rare earth metal-containing metallic magnesium adhering to the surface of the remaining magnet powder is replaced with metallic zinc, thereby separating the magnet powder from the rare earth metal-containing metallic magnesium (second separation step, ST209).

[0092] The rare earth metal-containing liquid magnesium separated in the second separation step is distilled to recover the rare earth metal (ST208). The distilled metallic magnesium is reused as a magnesium raw material in the dissolving step (ST204).

[0093] On the other hand, for the magnet powder and metallic zinc, the metallic zinc is distilled to recover the magnet powder (Fe—B) (ST210). The distilled metallic zinc is reused as a zinc raw material in the zinc substitution treatment step (ST209).

[0094] The rare earth metals recovered by distillation of metallic magnesium can be distilled in the order of increasing vapor pressure, namely, Dy distillation (ST211), Nd distillation (ST212), Pr distillation (ST213), and Tb distillation (ST214). Alternatively, only Dy distillation can be performed, and the residue can be sold as didymium. Alternatively, the residue can be sold by the above-mentioned residue drift method.

[0095] In this way, each rare earth element is recovered from the rare earth magnet. According to this embodiment, the rare earth metals dissolved in the liquid metallic magnesium separated in the first separation step and the liquid metallic magnesium separated in the second separation step are recovered, so that the efficiency of recovering rare earth metals from the rare earth magnet can be improved.

[0096] DESCRIPTION OF SYMBOLS 10... Processing device 11... Melting furnace 12... Container 13... Vacuum pump 14... Gas source 15... Stirring blade 17... Tilting shaft 19... Auxiliary container 20 Bubble ring nozzle

Claims

1. A method for recovering rare earth metals, comprising: a crushing step for crushing rare earth magnets; a dissolving step for dissolving rare earth metals from the magnet powder obtained in the crushing step into liquid metallic magnesium; a first separation step for separating the residue from the dissolving step from liquid metallic magnesium containing rare earth metals; a second separation step for replacing the liquid metallic magnesium containing rare earth metals attached to the residue from the dissolving step separated in the first separation step with liquid metallic zinc; and a recovery step for recovering rare earth metals by distilling each of the liquid metallic magnesium containing rare earth metals separated in the first separation step and the second separation step.

2. A method for recovering rare earth metals according to claim 1, wherein the crushing step includes a treatment for hydrogen embrittlement of the rare earth magnets.

3. A method for recovering rare earth metals according to claim 2, wherein the average particle size of the magnet powder is 1 μm or more and 150 μm or less.

4. A method for recovering rare earth metals as set forth in claim 3, wherein the average particle size of the magnet powder is 50 μm or more and 75 μm or less, and the maximum particle size is 90 μm or less.

5. A method for recovering rare earth metals according to claim 1, wherein the weight ratio of the liquid metallic magnesium to the magnet powder in the dissolving step is 0.36 or more and 5 or less.

6. A method for recovering rare earth metals according to claim 5, wherein the weight ratio of the liquid metallic magnesium to the magnet powder is 1 or more and 2 or less.

7. A method for recovering rare earth metals according to claim 5, wherein the atmosphere in the dissolving step is normal pressure or reduced pressure, and the temperature of the liquid magnesium metal is 650°C or higher and 1000°C or lower.

8. A method for recovering rare earth metals according to claim 6, wherein the dissolution treatment involves dissolving the rare earth metals from the magnet powder into the liquid metallic magnesium while flowing a mixture of the magnet powder and the liquid metallic magnesium in a furnace.

9. A method for recovering rare earth metals as set forth in claim 1, wherein the recovery step includes a process for separating light rare earth elements and heavy rare earth elements from the rare earth metals by causing them to drift due to differences in melting points.

10. A method for recovering rare earth metals as set forth in claim 9, wherein the recovery step comprises distilling metallic magnesium containing the liquid rare earth metals, and then heating the residue to a temperature equal to or higher than the melting point of Nd, thereby bleeding liquid phase light rare earth elements from a solid phase heavy rare earth element-containing material.

11. A method for recovering rare earth metals as set forth in claim 1, further comprising the steps of: prior to the dissolving step, heating the magnet powder obtained in the crushing step at a temperature of 400°C or higher and 650°C or lower; and after the dissolving step, recovering oxides of rare earth metals floating on the surface of the liquid metallic magnesium.

12. A method for recovering rare earth metals according to claim 11, wherein the rare earth metal oxide is a Dy oxide or a Tb oxide.

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